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Many years ago, I took a new apprentice into a mechanical room where an old sidewinder was running and showed him the control panel that was tight against a wall. I then explained how these machine were know to make a loud noise and then explode like a 2000 pound bomb, while all the time turning the load limit percentage lower and lower.
When it surged it scared him senseless.....That was as much fun as sending them into the air box and turning off the lights just as you started the huge fan which filled the room with belt squealing/rubber smoke...

As mentioned, a cent is not a positive displacement compressor. There are two forces at work on the refrigerant when it leaves the impeller. 1st is the force trying to move the refrigerant in a radial direction (direction away from the impeller). 2nd is the force trying to move the refrigerant in the direction of impeller rotation. Together these two forces generate a resultant velocity vector (lift) which is the amount of kenitic energy in the gas.
This lift is a fixed number (say 70 degrees) designed in an impeller. As long as you stay within that number you are safe. However; when when you try to go to go outside the design the wheel cannot lift the gas and will "stall" (quit pumping -surge). Indications are noise, amps falling and then rising, and oil pressure falling and rising. Surge is where the wheel quits pumping, lets the gas go backward through the wheel until the pressures even out a bit. and then gets another gulp of gas to try to pump again. This action applies to pumps, fans, and jet engines - all centrifugal loads.
Low load will not always cause surge (cents are used to make ice) as long as the head pressure drops with the suction. Conversly, high head will not always cause surge if the suction comes up too. It's when you have high head (for whatever reason) and low suction that you are probably outside the designed "lift" and the machine will protest.

So I can 'control' away surging?

As my load falls, and my suction pressure falls, if I controlled the condenser water, and head pressure to fall as well - the unit would never surge? Is that true?

Is there any sort of limitation of how low the head pressure can be?

PHM
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Originally Posted by Chiller Guy

As mentioned, a cent is not a positive displacement compressor. There are two forces at work on the refrigerant when it leaves the impeller. 1st is the force trying to move the refrigerant in a radial direction (direction away from the impeller). 2nd is the force trying to move the refrigerant in the direction of impeller rotation. Together these two forces generate a resultant velocity vector (lift) which is the amount of kenitic energy in the gas.
This lift is a fixed number (say 70 degrees) designed in an impeller. As long as you stay within that number you are safe. However; when when you try to go to go outside the design the wheel cannot lift the gas and will "stall" (quit pumping -surge). Indications are noise, amps falling and then rising, and oil pressure falling and rising. Surge is where the wheel quits pumping, lets the gas go backward through the wheel until the pressures even out a bit. and then gets another gulp of gas to try to pump again. This action applies to pumps, fans, and jet engines - all centrifugal loads.
Low load will not always cause surge (cents are used to make ice) as long as the head pressure drops with the suction. Conversly, high head will not always cause surge if the suction comes up too. It's when you have high head (for whatever reason) and low suction that you are probably outside the designed "lift" and the machine will protest.

PHM
--------
The conventional view serves to protect us from the painful job of thinking.

As my load falls, and my suction pressure falls, if I controlled the condenser water, and head pressure to fall as well - the unit would never surge? Is that true?

Is there any sort of limitation of how low the head pressure can be?

PHM
--------

Yes that is correct, providing that all other factors that can cause surging are ok.
Yes there is a limit on how low you can go, 65° F seems to come to mind on newer stuff. You won't get there if it's 90° F and 90% humidity outside.

But if the equipment is sized correctly that won't happen -

When it's 90 / 90 outside the unit has plenty of load and generous towers.

The problems come when the temperature are low. Like now. Years ago I told them to pipe for tower cooling but they didn't. So now they have a 400 ton centrifugal cooling a 150 ton winter-time load.

But the program logic is trying to control the condensing temp at the same point as in the summer. <g>

Thanks! I can make screw machines sing and dance and tell funny stories. But I am not a centrifugal guy. <g>

PHM
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Originally Posted by Carrene #2

Yes that is correct, providing that all other factors that can cause surging are ok.
Yes there is a limit on how low you can go, 65° F seems to come to mind on newer stuff. You won't get there if it's 90° F and 90% humidity outside.

PHM
--------
The conventional view serves to protect us from the painful job of thinking.

But the program logic is trying to control the condensing temp at the same point as in the summer.

That is basically your problem. When a centrifugal is unloaded, it does not have the ability to "lift" as much as when fully loaded. A constant speed machine will have its gas flow throttled by the inlet guide vanes. A variable speed machine will generally have the inlet guide vanes wide open, but the drop in speed drastically redcuces it's ability to "lift". Therefore, the condenser temperature has to come down and/or the evaporator temperature has to rise to reduce the "lift" required by the impeller.

All the newer chillers (last 20 years) have engineering specs. that show part-load capabilities and requirements. They will all show that, as the load drops, the lift must also drop, or it will go outside the surge envelope. Most all of them you will find call for 65 degree condenser water at 60% load.

If you don't have the origional part load engineering specs., The sales office that sold the machine should be able to get it for you.

Part of the confusion about surging is that the assumption is made that the load appears immediately and directly at the suction inlet. The machine's load control regulates the load by opening and closing vanes in the inlet stream of the wheel. This means the actual suction at the wheel is lower than the suction line pressure. Todays machines also have variable frequency drives that further complicate the issue. At to be blunt the controllers that ramp up and down the machine are frequently very stupid.

General rule of thumb w a centrifugal is don't do anything too quickly when you are playing with setpoints or load limits.

But the program logic is trying to control the condensing temp at the same point as in the summer. <g>

Thanks! I can make screw machines sing and dance and tell funny stories. But I am not a centrifugal guy. <g>

PHM
-------

I would say that I could make a centrifugal chiller sing and dance. LOL
I get along with the control companies very well (tech's are often personal friends), advantage of being in a rural area and have spent some amount of time training them on this topic. I get them to set the system up for spring and fall conditions and let the other conditions (summer) just maintain what it can, load increases along with condenser water temps natuarally.

condenser watersetpoint

the factory says you can go as low as 14 degrees above the leaving chilled water setpoint with the condenser water setpoint .!what they are telling you is keep the differential to a minimum ive seen machine s running at 30 hz 8% vane 19 kw .5 delta t and 10 degrees diff between leaving chwtr 42 and 52 degree leaving condenser water. had too listen hard too hear it run!
get the design flow right, and true ," keep it stable "and it will perform greatly later that day as the load increased the machine was able too track no problem.

condenser water setpoint

the factory says you can go as low as 14 degrees above the leaving chilled water setpoint with the condenser water setpoint . think about that!what they are telling you is keep the differential to a minimum ive seen machine s running at 30 hz 8% vane 19 kw .5 delta t and 10 degrees diff between leaving chwtr 42 and 52 degree leaving condenser water. had too listen hard too hear it run!
get the design flow right, and true ," keep it stable "and it will perform greatly